1. Field of the Invention
The present invention relates to an apparatus for selecting a terminal equipment in telephone lines connecting a telephone and a data transmitter at terminals by detecting the off-hook operation of the telephone during the operation of the data transmitter whereby the data transmitter is stopped and the telephone is selected to connect it to an exchange center.
2. Description of the Prior Arts
The circuit structure of the conventional apparatus is shown in FIG. 1.
In FIG. 1, the reference numeral (1) designates a network control unit (hereinafter referring to as NCU); (2) designates a data terminal equipment (hereinafter referred to as DTE) which is actuated by NCU (1); and (3) designates a telephone (TEL); (11) designates a circuit for detecting a NCU actuating signal (detection of calling); (12) designates a receiving relay which is self-sustaining by the output of the calling detection circuit (11); ra and rb respectively represent relay contacts; (21) designates an encoder-decoder circuit (CODEC); (22) designates a modulator-demodulator circuit (MODEM); (23) designates a constant voltage circuit (AVR).
The telephone lines L.sub.1, L.sub.2 are respectively connected to an exchange center (not shown). The exchange center comprises a DC power source for feeding power to the telephone lines L.sub.1, L.sub.2 and a switch for inversion of the output voltage of the DC power source.
The operation of the conventional apparatus will be illustrated.
The actuation of DTE (2) is carried out as follows.
The diode D.sub.1 is turned on to connect NCU (1) to the power source, by the inversion of the telephone lines L.sub.1 and the L.sub.2 (plus at L.sub.2 side) which is caused by a calling signal from the exchange center. Then, the calling signal is input through the coupling capacitor C.sub.1 into the calling detection circuit (11). The calling signal is detected by the calling detection circuit (11) to turn on its output transistor. Then, the transistor TR.sub.1 is turned on to actuate the receiving relay (12). When the receiving relay (12) is once actuated, the self-sustaining current is fed to the relay contact rb thereof whereby the operation can be continued.
During the period of actuating the receiving relay (12), power is fed to DTE (2) and MODEM (22) is connected to the lines so as to be capable of transmission and receiving.
When an off-hook of the telephone (3) is carried out during the period of actuating DTE (2) (hook switch HS in the closing state), a step for detecting the off-hook state to stop the operation of DTE (2) is as follows. That is, the current consumed in NCU (1) and DTE (2) is remarkably smaller than the telephone current of the telephone (3). When the off-hook of the telephone (3) is carried out, the receiving relay (12) is returned to stop the operation of DTE (2) under the condition of an apparent short-circuit between the power input terminals of NCU (1) and DTE (2) as produced by the low resistance of the telephone (3).
The conventional apparatus has said structure. Thus, it is necessary to control precisely the coil current of the receiving relay (12) in the range of a non-induction current during the period of the off-hook of the telephone (3).
However, several phenomena have been observed, as follows:
(a) a fluctuation of the voltage between the terminals of the telephone is large during the period of the off-hook of the telephone because of the fluctuation of the line impedance.
(b) A fluctuation of the non-induction current of the relay is large.
(c) The current consumed by NCU (1) and DTE (2) should be remarkably small.
It has been difficult to attain stable operation because of said reasons.